Generally, aluminum and aluminum alloys are formed into a desired shape for application by means of rolling, pressing and the like. For example, a container of aluminum or aluminum alloy (hereinafter referred to as “aluminum container”) is usually formed into a can by means of drawing, which is referred to as drawing and ironing. The forming lubricant adheres on the surface of the aluminum cans shaped by the forming method mentioned above. Additionally, such contaminants as fine powder generated during the forming and the like adhere on the surface. These contaminant materials are inconvenient for the subsequent surface treatments and paint coating and hence are removed by various cleaning. It is known that superiority or inferiority of the cleaning property exerts significant influence upon the surface treatments and quality of the paint coating.
The cleaning liquid, which is commercially used at present to clean the aluminum container, is a sulfuric-acid aqueous solution, which contains hydrofluoric acid and one or more surfactants, or an aqueous solution, which contains phosphoric acid, nitric acid, ferric iron and sulfuric acid, as well as one or more surfactants. These acidic cleaning liquids are very effective and have a number of advantageous features.
However, the equipment of the cleaning line of aluminum formed articles usually made of stainless steel and other steels is disadvantageously corroded by these acidic cleaning liquids. Manpower and high cost are, therefore, necessary for the maintenance of the cleaning line. In addition, the waste liquid containing hydrofluoric acid and aluminum fluoride raises environmental problems in the treatment of waste liquid containing fluorine. Furthermore, the cleaning liquid, which contains trivalent iron necessitates treatment at high temperature exceeding 70° C. and raises energy problem.
Heretofore, several alkaline cleaning liquids for the aluminum container have been proposed to solve the problems described hereinabove. In Patent Documents 1 through 5, particular compositions of the alkaline cleaning liquid have been proposed, but the industrial property is unsatisfactory. It has also been proposed to pickle after alkaline cleaning or to alkaline clean with the proviso that a neutralizing process using acidic liquid is carried out. In every case, mass production technique in industry scale is not at all established. This is because, although the pickling process introduced subsequent to the alkaline cleaning dissolves a thickly grown oxide film in the alkaline cleaning and is effective and necessary for preventing discoloration and improving the paint adherence, the existing plant and the like imposes a limitation on the use of pickling.
The alkaline cleaning liquid proposed in Patent Document 5 has a particular composition consisting of one or more alkaline builders, at least one compound selected from aminoalkyl diphosphonic acid and hydroxyalkyl diphosphonic acid, and at least one aluminum-ion blocking agent selected from alkali metal salt of gluconic acid, alkali metal salt of oxalic acid, alkali metal salt of tartaric acid, or sorbitol, and surfactant. The technique proposed is to suppress growth of oxide film on the surface of an aluminum container to be cleaned or to suppress the segregation of Mg. The pickling process is not necessary.
However, this technique does not intend to apply to industrial continuous production. Incidentally, no appropriate method of controlling alkaline cleaning liquid capable of practical application has been provided. Therefore, the level of technique at the time of 1993 roughly targeted the use the alkalinity and surface tension (Non Patent Document 1).
However, it was discovered that the alkaline cleaning liquid disclosed in Patent Document 5 is strongly influenced by the particular metal-ion component incorporated in the liquid. It was discovered that the behavior and condition of the particular metal-ion components during the industrial continuous production are as follows. Specifically, the particular metal-ion components are already present in the water used for the cleaning liquid or dissolve from the aluminum alloy. The amount of particular metal-ion components varies. Variation in the amount of the particular metal-ion components is the reason why etching-stability and uniformity cannot be continuously maintained. Particularly, local corrosion of the aluminum container is liable to occur in the cleaning (hereinafter referred to as “pitting corrosion”). This is a serious drawback that may cause the can's flange to crack.
In addition, in the production line, the alkaline cleaning liquid may be discarded or replenished at the time of plant maintenance and the like. After discarding, alkaline cleaning liquid is prepared fresh and the continuous production is started. Along with treatment of aluminum alloy, a constant portion of the alkaline cleaning liquid is discarded, and fresh cleaning liquid, which compensates for the discarded amount, is added in the continuous production. During the alkaline cleaning, the particular metal-ion components, which dissolve along with treatment of aluminum alloy, continuously increase and then reaches in the saturation state in a certain period. The period, in which the concentration of particular metal-ion components arrives at saturation state in the alkaline cleaning liquid, is determined by the size of a vessel for storing the alkaline cleaning liquid is, and the discarding and replenishing amounts mentioned above. In most of the lines, approximately 10 hours is necessary to reach the saturation state. There is a possibility that the concentration of particular alloy-components varies during 10 hours mentioned above, so that the constant property is not attained. Several of the treated aluminum containers may not be commercially accepted. The aluminum containers produced in such period may be 500,000 or more, which is a very disadvantageous production result.
Patent Document 1: Japanese Unexamined Patent Publication (kokai) Sho 59-133382
Patent Document 2: Japanese Patent No. 2587916
Patent Document 3: Japanese Unexamined Patent Publication (kokai) Sho 62-247090
Patent Document 4: Japanese Unexamined Patent Publication (kokai) Sho 62-182291
Patent Document 5: Japanese Unexamined Patent Publication (kokai) Hei4-187788
Non-patent Document 1: NP Series “Surface Cleaning Techniques” published by Maki Shoten on Nov. 10, 1993 (first edition) and Oct. 10, 1998 (first edition and second print), page 91